1. Field of the Invention
The present invention is related to a helical antenna used in a mobile wireless (radio) appliance such as a portable telephone.
2. Description of the Related Art
Very recently, mobile communications, e.g., portable telephones are rapidly developed. Not only ground mobile communication systems are available, but also satellite mobile communication systems are expected for practical uses. In such mobile communication terminals, antennas may constitute one of the major important devices, or components.
Now, one example of conventional 4-winding helical antennas will be described with reference to drawings. FIG. 11 schematically shows an electric power feeding circuit for this conventional helical antenna, and FIG. 12 is a plan view of the helical antenna to which electric power is supplied by employing the feeding circuit.
An (electric power) feeding circuit 200 is provided with a 3dB-hybrid circuit 201, a balun circuit 202, and another balun circuit 203. These circuits 201 to 203 are mounted, or packaged on the same plane of a mounting board 204 under such a condition that these circuits 201 to 203 are connected via a strip line having a resistance value of 50 .OMEGA. to each other.
The hybrid circuit 201 is a circuit for producing an output signal whose output phase is in phase with the input phase thereof (will be defined as a "0.degree. output" hereinafter), and another output signal whose output phase is delayed by 90.degree. from the input phase thereof (will be defined as a "90.degree. output" hereinafter) from a signal which is supplied to the antenna for feeding the electric power. It should be noted that an output signal whose output phase is delayed by 180.degree. from the input phase thereof is defined as a "180.degree. output", and an output signal whose output phase is delayed by 270.degree. from the input phase thereof is defined as a "270.degree. output".
The balun circuit 202 contains a signal output unit 205 and another signal output unit 206. The 0.degree. output derived from the hybrid circuit 201 is entered into the signal output circuit 205 and the signal output circuit 206, respectively. The signal output units 205 and 206 produce both the 0.degree. output and the 180.degree. output with respect to this input signal of the 0.degree. output as feeding signals, and then output these feeding signals.
The balun circuit 203 contains a signal output unit 207 and another signal output unit 208. The 90.degree. output derived from the hybrid circuit 201 is entered into the signal output circuit 207 and the signal output circuit 208, respectively. The signal output units 207 and 208 produce both the 0.degree. output and the 180.degree. output with respect to this input signal of the 90.degree. output as feeding signals, and then output these feeding signals.
As a consequence, the relationship among these feeding signals is established as follows: That is, with respect to the 0.degree. output of the signal output unit 205, the 180.degree. output derived from the signal output unit 206 is delayed by 180.degree.; the 0.degree. output derived from the signal output unit 207 is delayed by 90.degree.; and the 180.degree. output derived from the signal output unit 208 is delayed by 270.degree..
In a helical antenna 210, 4 pieces of antenna elements (not shown) are arranged in a helical form along an outer surface of a hollow cylindrical body 211.
Each of the antenna elements owns each of signal input units 212 to 215. The respective signal input units 212 to 215 are arranged in an equi-interval of 90 degrees on an edge portion of the cylindrical body 211, and also are connected to the respective signal output units 205 to 208 via a power feeding line 216 made of a conductive line with maintaining an individual relationship among them.
As a result, the power feeding signals are supplied from the feeding circuit 200 to the respective antenna elements under such a condition that the phase differences among these feeding signals are made by 90 degrees.
On the other hand, the signal input units 212 to 215 of the respective antenna elements are arranged on an edge surface of the cylindrical body 211, namely on a circumference within the same plane.
However, the respective signal output units 205 to 208 of the feeding circuit 200 are arranged on the same straight line at an edge portion on the mounting plane of the board 204.
As a result, the connection distances "a" to "d" between the signal output units 205 to 208 and the signal input units 212 to 215 are made incoincident with each other.
In the case of the antenna arrangement shown in FIG. 12, the connection relationship is given by d&gt;a.congruent.b&gt;c. In particular, a distance difference between a connection distance "c" (interval between 207 and 213) and another connection distance "d" (interval between 208 and 215) becomes large.
As previously explained, while the connection distances "a" to "d" are made incoincident with each other, if the signal output units 205 to 208 are connected to the signal input units 212 to 215 by way of the feeding lines 216(a) to 216(d), then a large difference is produced in the lengths (electric lengths) of the feeding lines 216(a) to 216(d).
As a consequence, the feeding signals having the phase differences by 90 degrees are not originally supplied to the respective antenna elements. Accordingly, the axial ratio of the radiated circularly-polarized wave is increased. Furthermore, the horizontal plane directivity of this helical antenna is deteriorated. As a result, the signal transmission/reception cannot be carried out in high precision.